From the Super Mario Wiki
The Hard Court is a tennis court appearing in all Mario Tennis games. It is the default court, and is the court used in the Mushroom Cup in Mario Tennis. Hard courts are the most commonly used tennis courts in real life.
In game the court is described as "A standard composite court with average ballspeed." The court's stats say it has normal ballspeed and a strong bounce. It is a good court for beginners and is the most commonly used court.